Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
For pulsed radar level gauge systems, time expansion techniques are generally used to resolve the time-of-flight.
Such pulsed radar level gauge systems typically have a first oscillator for generating a transmission signal formed by pulses for transmission towards the surface of the product contained in the tank with a transmitted pulse repetition frequency ft, and a second oscillator for generating a reference signal formed by reference pulses with a reference pulse repetition frequency fr that differs from the transmitted pulse repetition frequency by a given frequency difference Δf. This frequency difference Δf is typically in the range of Hz or tens of Hz.
At the beginning of a measurement sweep, the transmission signal and the reference signal are synchronized to have the same phase. Due to the frequency difference Δf, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep.
During the measurement sweep, the reflection signal formed by the reflection of the transmission signal at the surface of the product contained in the tank is being correlated with the reference signal, to form a measurement signal based on a time correlation between the reflection signal and the reference signal. Based on the measurement signal, the filling level can be determined.
Typically, oscillator regulation of the second oscillator in relation to the first oscillator in the radar level gauge system is based on the frequency difference Δf. However, due to the low frequency of the frequency difference Δf, the response of the oscillator regulation is slow and cannot in a suitable manner control, or mitigate, frequency disturbances faster than the frequency difference Δf.
In order to provide a faster response of the oscillator regulation, it is known to increase the oscillator frequency of one or both of the oscillator in order to provide a frequency difference Δf signal with a higher frequency, and then divide the oscillator frequency down to the operating frequency, such as is, for example, disclosed by U.S. Pat. No. 6,072,427.
Although providing for more frequent regulation of the frequency of the reference signal, the higher frequency oscillator(s) results in an increased power consumption.